This invention relates to the preparation of elastomeric, urethane/urea copolymers. More particularly, this invention relates to novel compositions for preparing elastomeric urethane/urea copolymers wherein the reagents employed to prepare the copolymers include a difunctional amine as the chain extender and a trifunctional amine as a crosslinking agent.
Elastomeric urethane/urea copolymers are conventionally prepared by reacting an isocyanate-terminated prepolymer (obtained by reacting a polyol having a functionality of about two and a stoichiometric excess of a diisocyanate compound) with a difunctional amine as the chain extending agent, optionally in the presence of a suitable catalyst to obtain final elastomer. The elastomer is believed to be a block copolymer containing both urethane and urea groups.
Up until recently the preferred chain extender was 4,4'-methylene-bis(2-chloroaniline), which is referred to in the trade as "MOCA." MOCA imparts excellent mechanical strength properties to urethane elastomers, however, the use of this diamine has been severely restricted since regulations appeared in the Federal Register, Vol. 38, No. 144, July 27, 1973. These regulations require that strict precautions be exercised during the manufacture, handling and use of MOCA. These restrictions have encouraged a search for other chain extending agents which impart the same level of desirable physical properties as MOCA.
U.S. Pat. No. 3,920,617 teaches that sulfur-containing diamines of the general formula ##STR1## wherein R is alkylene and R' is hydrogen, halogen or a hydro-carbon group are useful chain extending agents for urethane type elastomers. The rate at which this class of compounds reacts with prepolymers terminated using 2,4- and 2,6-tolylene diisocyanates to form a molded article that retains its shape and structural integrity is considerably slower than the rate achieved using other prior art amine-type curing catalysts such as MOCA. A slower reaction is a considerable disadvantage in a commercial molding operation, since it would increase the time required for the material to become sufficiently cured that it can be removed from the mold. A mixture of isocyanate-terminated prepolymer and diamine curing agent is usually a liquid of from moderate to high viscosity. The viscosity gradually increases as these two materials react. This liquid is poured into a heated mold wherein the liquid cures to form a solid material. The time interval between filling of the mold and the earliest time at which the molded object will retain its shape when removed from the mold is known as the "demold time." It is highly desirable to achieve the shortest possible demold time, since this will permit an increase in production rate with a corresponding decrease in production costs. The rate at which the isocyanate reacts with the prepolymer to form a non-flowing material should be slow enough at allow the reaction mixture to fill all of the mold cavities.
It is an objective of this invention to decrease the time interval required to convert prepolymers derived from a difunctional isocyanate and a polyol to a non-deformable and demoldable state using any of the sulfur-containing diamine-type chain extending agents disclosed in the aforementioned U.S. Pat. No. 3,920,617. It has now been found that this objective can be achieved using certain trifunctional amines as crosslinking agents in conjunction with these sulfur-containing aromatic diamines.